1. Field of the Invention
The present invention is directed generally to electric motors and more particularly to an electric motor providing full utilization of haromonic energy as well as electronically switched stator field current and integral torque control.
2. Description of the Prior Art
It is well known that conventional electric motors are not designed to respond efficiently to nonsinusoidal driving currents. In the usual case, a synchronous motor driven by a sinusoidal power source will run at a fixed speed corresponding to the line frequency and motor configuration. However, if a nonsinusoidal driving current containing harmonic is used energy is lost without resulting in useful work output at the rotor shaft. In some cases nonsinusoidal driving currents can cause excessive motor vibration or even a reduction in useful output torque. In addition, the unutilized harmonic energy may also appear as heat within the motor which can lead to shortened life and increased maintenance costs. Such problems have often confronted designs of electronically switched motor currents which often must employ nonsidusoidal switching currents.
In addition, schemes for improving the efficiency of or reducing the materials utilized in AC or DC motors are well known and numerous. In recent years, attention has been particluarly directed to eliminating the need to mechanically commutate or switch large field currents in such motors. Mechanical commutation has generally brought with it mechanical wear and increased noise and electrical interference caused by electrical transients produced during the switching process. In addition, mechanical commutators and their associated support assemblies add substantial weight and rotating mass to the motor, as well as create the need for interpoles and the like to correct inefficient field patterns.
Prior art workers have suggested various types of electronic switching techniques to replace the mechanical commutation, and have in some case relied upon optical sensing of rotor position to fix the field current switch points. However, most of these approaches fail to fully utilize the applied energy as described hereinabove, resulting in inefficient operation and unnecessarily complex and rugged construction.
Furthermore, existing electronic motor control systems have not provided a satisfactory means for effectively controlling stator field current to modify the torque characteristics of the motor. This deficiency has led to the existence of a large number of custom designed motors adapted to a particular application, rather than a motor of universal application with variable torque characteristics which can be easily and inexpensively adapted to a particular working environment.